1. Field of the Invention
The present invention relates to a method of selectively modifying the physical characteristics of structural members to improve control of part deformation and enhance impact energy absorption.
2. Background Art
Certain structural components of vehicles are designed to improve crash worthiness of vehicles in the event of a collision. Impact energy absorption characteristics of vehicle parts are engineered to provide improved protection of vehicle occupants. For example, bumpers, bumper support brackets, steering columns, drive shafts and the like are designed, in part, to withstand the impact of a collision and absorb impact loads through material deformation.
It Is preferable that some vehicle parts bend in a controlled manner from the standpoint of improving impact energy absorption because bending the part allows a part to absorb more energy than if the part breaks or buckles in a single location. Passenger compartment pillars, steering wheels and engine mounts are parts that benefit from controlling deformation in the event of a collision.
Protective crumple zones and selectively weakened structural components are known to be enhanced by drilling, notching or otherwise reducing the cross-sectional area of component parts. Forming holes or slot to enhance the crush behavior structural components adds to tooling and manufacturing costs. When structural members are drilled, pierced or otherwise modified by conventional methods, stress risers are created and work hardening of the metal may lead to fatigue and tend to enhance the likelihood that a part will be susceptible to fatigue cracks and breakage. Providing holes or notches In impact absorbing members may also increase susceptibility to corrosion.
Another method of controlling crushing is to provide convolutions or ridges in an energy absorbing member. This alternative requires expensive manufacturing steps and does not facilitate the use of low-cost extruded parts.
Parts that are too strong are capable of transmitting higher loads and are not easily crushed to absorb impacts. Structural strength obtained by using high strength alloys of aluminum, steel and other metals permits reduction in the weight of component parts. However, increased strength of such parts causes them to become more susceptible to fracture and less capable of absorbing impact loads through deformation.
There is a need for a simple and Inexpensive method for improving the impact energy absorbing characteristics of structural vehicle parts. There is also a need for structural components made from high strength alloys that yield to impact forces in a controlled manner. A need also exists for flexible design alternatives that can be tuned for impact energy absorption and controlled deformation. Tuning the impact energy absorbing characteristics and deformation control characteristics of a part is not easily accomplished with prior art techniques and could only be provided with extensive tooling and complex manufacturing processes. Other characteristics that are important to improve include corrosion resistance, durability and improved noise, vibration and harshness.
The above problems and shortcomings of the prior art products and methods are addressed by applicants"" invention as summarized below.
According to one aspect of the present invention, a crush rail having improved structural properties for Improving the crash worthiness of a vehicle is provided. The crush rail comprises an elongated rail made from metal that is provided with a crush trigger. The crush trigger is integrally formed on the rail and is a localized part of the elongated member that is heat treated to decrease the yield strength and increase the ductility of the metal. The localized part of the elongated member is spaced from an end of the rail.
According to other aspects of the invention, the crush trigger is heat treated by means of induction heating. The elongated rail may be made from aluminum, steel, or magnesium alloys.
According to another aspect of the invention, a bumper support bracket having improved crash worthiness is provided that comprises an elongated metal member with a rectangular cross section. The elongated member has a first end bolted to the frame of the automobile and a second end spaced from the frame. At least one crush trigger is formed by heat treating a localized area spaced from but near the second end of the elongated member. The crush trigger is an area of reduced yield strength and increased ductility.
According to other aspects of the invention as it relates to a bumper support, the elongated rail is made from a heat treatable metal alloy. The crush trigger may be heated circumferentially by various heating processes, for example, by induction heating. The crush trigger is created at a pre-determined distance from the free edge with the distance being determined as a function of the material characteristics, wall thickness and dimensions of the rectangular tube. The crush trigger is located to control the location and force required to initiate crush upon impact.
A plurality of crush triggers may be provided in addition to the first crush trigger with a distance between each crush trigger being substantially the same distance as the distance between the second end and the first crush trigger. The first crush trigger is preferably heat treated to obtain a particular yield strength and a particular ductility. Each subsequent crush trigger may have a relatively higher yield strength and relatively lower ductility than the crush trigger adjacent to the crush trigger in the direction of the second end.
According to another aspect of the invention, a body pillar assembly for a vehicle is provided. The body pillar includes an elongated pillar formed of a heat treatable alloy. The pillar has a bottom end attached to the chassis or compartment pan of the vehicle and a top end attached to the body of the vehicle at a point spaced above the chassis of the vehicle. At least one crush trigger is formed by heat treating a localized area of the pillar to locally reduce yield strength and increase ductility of the pillar.
According to other aspects of the invention, as they relate to body pillars of a vehicle, the crush trigger may be created by means of induction heating. For example, one crush trigger may be provided near a top end of the b-pillar and a second crush trigger may be provided proximate the bottom end of the b-pillar which is attached to the chassis or compartment pan.
According to another aspect of the invention, a steering column assembly of a vehicle having improved crash worthiness and steering rigidity is provided. The steering column assembly includes a steering column component made from a heat treatable tube having a circular cross section. The steering column component has a first end that is closest to the steering wheel and a second end that is closest to the steering gear. A crush trigger is created by locally heating an area of the heat treatable tube to decrease the yield strength and increase the ductility of the metal. The crush trigger is formed-near the end of the heat treatable tube that is attached to the steering gear.
According to another aspect of the invention, a drive shaft for a rear wheel drive vehicle is provided. The drive shaft includes an elongated member made from heat treatable metal having a circular cross section. The drive shaft includes two ends with one end being attached to a transmission and the other end being attached to a rear differential assembly. A crush trigger is created by heat treating the metal to decrease the yield strength and increase the ductility of the metal. The crush trigger is formed near the end of the drive shaft that is connected to the rear differential assembly. The drive shaft may be made by an extrusion process.
According to another aspect of the invention, an engine support frame, or mount, is provided for improving the crash worthiness of a vehicle. The engine mount may be made of a heat treatable metal with a crush trigger being created by forming a hole in the engine mount and heat treating an area rearward of the hole in a localized area to reduce the yield strength and increase the ductility thereof. In the event of an impact that involves the engine, a fastener securing the engine to the engine mount can deform the localized area on the engine mount and elongate the hole that receives the fastener.
Another potential application of the invention is in the manufacture of steering wheels. By using the present invention a steering wheel can be made of a high strength alloy to obtain weight savings. The steering wheel would then be locally heat treated to reduce its strength around a portion of its periphery so that the wheel would deform in the event of a collision that would cause a driver""s head to impact the wheel. Another advantage of this application is that the rigidity of the steering wheel is improved.
According to another aspect of the present invention, a crush rail is prepared from an elongated member made from a heat treatable metal having a first and a second end and intermediate portions between the first and second ends. A crush trigger is created by heat treating the elongated member with a continuously varied yield strength and ductility being provided along the length of the elongated member to provide progressively greater yield strength and reduced ductility from the first end to the second end of the elongated member. The yield strength and ductility are modified by heat treating different areas of the elongated member for different amounts of time. Yield strength and ductility may also be modified applied along the length of the elongated member by changing heat treat temperature or duration of heat treatment along the length of the elongated member. The first end is heat treated to have the lowest yield strength and the highest ductility while the second end has the highest yield and the lowest ductility with the yield strength of the intermediate portion of the elongated member varying continuously from the first end to the second end.
The above aspects of the different embodiments of the present invention and additional aspects will be better understood in view of the attached drawings and following detailed description of the various embodiments of the invention.